Evolution of radiation-induced lattice defects in 20/25 Nb-stabilised austenitic stainless steel during in-situ proton irradiation

Highlights: •Proton-irradiated cladding steel develops high dislocation density at low dpa levels. •Proton-induced $\frac{a_0}{3}〈111〉$ Frank loops and black spots saturate at only 0.1dpa at 420 °C. •The ratio of unfaulted to faulted loops remains close to 0.8 up to 0.8dpa at 460 °C. •At 500 °C the unfaulted dislocation loops transit into dislocation lines at 0.2dpa. •Voids are present at 0.8dpa with average size of 3.7 nm (460 °C) and 6.1 nm (500 °C). - Abstract: We have monitored in situ the lattice defect evolution induced by proton irradiation in 20Cr-25Ni Nb-stabilised stainless steel, used as fuel cladding material in advanced gas-cooled reactors. At 420 °C, the damaged microstructure is mainly characterised by black spots and faulted $\frac{a_0}{3}〈111〉$ Frank loops. Defect saturation is reached at only 0.1dpa. In contrast, at 460 °C and 500 °C proton bombardment induces the formation of a mixture of $\frac{a_0}{3}〈111〉$ Frank loops and perfect $\frac{a_0}{2}〈110〉$ loops. These perfect loops evolve into dislocation lines that form a dense network. This transition coincides with the saturation in the dislocation loop size and number density at 0.8dpa (460 °C) and 0.2dpa (500 °C), respectively. The presence of a high density of dislocation loops and lines at those two temperatures causes a vacancy supersaturation in the matrix, leading to the formation of voids and stacking fault tetrahedra.